1. Field of the Invention
The present invention relates to antenna devices and chip antennas. In particular, the present invention relates to an antenna device used for mobile communication, cellular communication, local area networks (LAN), television, radio, etc. Further, the present invention makes use of a chip antenna of small size, itself a monopole antenna, which, together with a ground part, functions like a dipole antenna, even though it is of small size.
2. Description of the Related Art
FIG. 3 shows a prior art monopole antenna 50. The monopole antenna 50 has a conductor 51, one end 52 of the conductor 51 being a feeding point and the other end 53 being a free end in the air (dielectric constant .epsilon.=1 and relative permeability .mu.=1).
Because the conductor of the antenna is present in the air in linear antennas, such as the prior monopole antenna 50, the size of the antenna conductor becomes larger. For example, when the wavelength in the vacuum is .lambda..sub.0 in the monopole antenna 50, the length of the conductor 51 must be .lambda..sub.0/ 4. Thus, such an antenna cannot be readily used for mobile communication or the like which requires a compact antenna.
Conventionally, as a small-sized antenna to be used in a radio equipment, an inverted F-type antenna is known. One example of an inverted F-type antenna is explained in reference to FIG. 11. The inverted F-type antenna 50 is composed of a printed-circuit board 52 which is made of a glass-filled epoxy resin of a relative dielectric constant of 4 to 5 and on a surface of which a ground electrode 51 connected to the ground electric potential is provided, and a radiator plate 53 which is made of a metal plate arranged in parallel with the printed circuit board 52 and above the printed-circuit board 52. The radiator plate 53 fulfills the function of radiating a radio wave, and its length is .lambda./4 (.lambda.: wavelength of the radio wave). On the side edge of the radiator plate 53, a short pin 54 extended toward the printed-circuit board 52 is integrally provided with the radiator plate 53. The short pin 54 is electrically connected to the ground electrode 51 on the printed-circuit board 52. That is, the radiator plate 53 is short-circuited to the ground electrode 51 through the short pin 54. On the printed-circuit board 52, a coaxial cable connection portion 52a is provided, and to the coaxial cable connection portion 52a a coaxial cable, a connector, etc. (not illustrated) through which a load dispatching to the radiator plate 53 takes place, are connected through a connection terminal 53a led out from the radiator plate 53.
However, in the above described inverted F-type antenna, there were cases, in which in order to realize a small-sized antenna, a dielectric substance is inserted between the ground electrode on the printed-circuit board and the radiator plate, and the wavelength shortening effect of the dielectric substance is used. In these cases there is a problem that the antenna gain is decreased because of the effect of the dielectric substance.